In late 1986 Bednorz and Muller disclosed that certain mixed phase compositions of La--Ba--Cu--O appeared to exhibit superconductivity beginning at an onset temperature of about 30.degree. K., over 7 degrees above the critical temperature of known Nb.sub.3 Ge compositions. Bednorz et al., Z. Phys. B., Condensed Matter, Vol. 64, pp. 189-198 (1986). The superconducting composition was determined to have a crystalline structure like that of K.sub.2 Ni.sub.1 F.sub.4, and is therefore referred to as a 214 composition. It has since been determined that whatever the rare earth metal or the alkaline earth metal constituent of such a 214 system may be, the upper temperature limit of superconducting onset, T.sub.co, of superconductors of a 214 type crystalline structure is no greater than about 38.degree. K. Liquid helium is still required as the coolant for such a 214 type of material.
Following the discovery of superconductivity in a rare earth-alkaline earth-Cu oxide system of a 214 crystalline structure, a new class of rare earth-alkaline earth-copper oxides was discovered which are superconductive at temperatures above the boiling point of liquid nitrogen, 77.degree. K. These new rare earth-alkaline earth-copper oxides are now commonly referred to as "123" high-temperature superconductors (HTS) in reference to the stoichiometry in which the rare earth, alkaline earth, and copper metal atoms are present, namely a ratio of 1:2:3.
The unit cell formula of the 123 HTS compounds is L.sub.1 M.sub.2 Cu.sub.3 O.sub.6+.delta. (.delta.=0.1 to 1.0, preferably about 1.0) wherein the constituent, L, is yttrium, lanthanum, neodymium, samarium, europium, gadolinium, dysprosium, holmium, erbium, thulium, ytterbium, or lutetium, or mixtures thereof including mixtures with scandium, cerium, praseodymium, terbium and the alkaline earth constituent, M, is barium, strontium or mixtures thereof. Among this class of 123 HTS compounds, that most preferred is Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.6 +.delta..
The 123 HTS compounds have a perovskite related crystalline structure. The crystalline unit cell of such 123 compounds consists of three sub-cells in alignment along the crystallographic C-axis wherein the center of the middle subcell is occupied by a rare earth metal atom (L), the center of each end subcell is occupied by an alkaline earth metal atom (M), the corner positions of each subcell are occupied by copper atoms, and intermediate the copper atoms along the edges of the subcells are sites for occupation by oxygen atoms. X-ray and neutron powder diffraction studies indicate the structure of superconductive 123 compounds to be oxygen deficient and that the ordering of oxygen in the basal planes is critical to the existence of superconducting properties in such compounds. See C. Poole et al, Copper Oxide Superconductors (John Wiley & Sons 1988).
Studies indicate that when 8 is between about 0.1 to about 0.6, the resulting 123 compound assumes a tetragonal unit cell crystallographic symmetry and is non-superconductive. In the tetragonal unit cell symmetry, the lattice dimension of the c axis is approximately 11.94 angstroms and that of the a and b axes is equal, approximately 3.9 angstroms. When .delta. is between 0.7 and 1.0, the resulting 123 compound has an orthorhombic unit cell crystallographic symmetry and is superconductive. The orientation of the oxygen atoms in the orthorhombic unit cell causes the unit cell to compress slightly along the a crystallographic axis and thus the lattice dimension of the a axis is less than that of the b axis. Lattice constants in the orthorhombic symmetry are about a=3.82, b=3.89 and c=11.55 angstroms.
As with the Nb.sub.3 Ge and 214 compositions, the new 123 compositions will, when maintained below the critical temperature (T.sub.c), exhibit resistance when subjected to an electrical current through a unit area above a certain amperage known as the critical current density (J.sub.c).
A 123 HTS compound, whether produced by solid state reaction, coprecipitation or by a sol-gel technique has, as an intrinsic property, a T.sub.c of .gtoreq.77 K. However, the J.sub.c of a body of a 123 HTS compound is highly dependant on the methodology used to produce the 123 HTS compound body. Accordingly, the process by which an article of 123 HTS is formed--whether that article is in wire, ribbon, film, rod or plate form--dictates many of the practical uses to which that 123 HTS article may be put, dependent upon the J.sub.c of the article.
For use in superconducting magnets, it is most efficient and desirable to produce a 123 HTS body article in a wire, ribbon, plate or cylindrical form. A wire/ribbon form enables the convenient fabrication of the wire/ribbon coils of the superconducting magnet. Yet, to have commercially practical application for superconducting magnets, the wire/ribbon in which a 123 HTS compound is produced must have in the maximum field of the magnet a J.sub.c at a minimum order of 10.sup.3 A/cm.sup.2. For commercially practical applications in power transmission lines, the wire/ribbon needs to have a J.sub.c of at least about 10.sup.3 to about 10.sup.5 A/cm.sup.2 in a magnetic field of about 1 T.
The superconductivity of an ideal orthorhombic 123 lattice network is anisotropic and it has been determined that the critical current density J.sub.c is greatest when measured along an axis in the ab plane of the unit cell. However, articles composed of the orthorhombic compositions which have been produced by solid state reaction are granular and though containing grains with a near perfect lattice structure, the grains of the aggregate material are poorly aligned. Thus, articles composed of such sintered compositions are isotropically superconductive and exhibit current densities below that required for commercial use in superconducting magnets and transmission lines.
Anisotropic superconductivity has been attained on a macroscopic scale in cold pressed and sintered forms of 123 superconductors produced by solid state reaction through the process of melt-texturing wherein the form is heated beyond the peritectic temperature of the superconducting composition to incongruently melt the composition into L.sub.2 M.sub.1 Cu.sub.l O.sub.5 and a liquid phase and cooled at a controlled rate to precipitate out grains of the superconducting material. The temperature profile of the cooling step is critical to the morphology of the precipitated grains, and reference is made to U.S. Pat. No. 4,956,336 for a more thorough explication of this aspect of the melt-texturing process. The effective application of the melt-texturing process causes the precipitation of highly aligned grains, which imparts to the bulk article composed of the precipitated grains anisotropic superconductivity approaching that of the ideal lattice network.
Due to the slow kinetics of melt-texturing methods, fabrication of large anisotropically superconducting articles is time consuming. Typical growth rates of the aligned grain structure are on the order of a few millimeters per hour. In addition, large articles require stringent processing conditions to maintain uniform grain alignment throughout the article. Overcoming these constraints on the size to which an article of 123 superconductor can be produced to have anisotropic superconductivity.
Preparation of larger articles from segments of melt-texturized segments wherein the larger article has comparable J.sub.c properties to those of the segments of which it is composed, would be one means for overcoming the size constraints. Joining segments of bulk superconductor would provide a way by which the attractive properties of melt processed materials can be extended to larger sizes and different shapes. For example, in addition to the possibility of fabricating long conductors capable of carrying high currents, superconducting magnets with uniform field profiles over larger areas could be made for bearing applications.